1. Field of the Invention
This invention relates to an improved process combination for the conversion of hydrocarbons, and more specifically for the upgrading of a naphtha stream by a combination of selective isoparaffin synthesis, etherification of light products, and reforming.
2. General Background
The widespread removal of lead antiknock additive from gasoline and the rising fuel-quality demands of high-performance internal-combustion engines have compelled petroleum refiners to install new and modified processes for increased "octane," or knock resistance, in the gasoline pool. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher-severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, isomerization of light naphtha and the use of oxygenated compounds. Such key options as increased reforming severity and higher FCC gasoline octane result in a higher aromatics content of the gasoline pool, through the production of high-octane aromatics at the expense of low-octane heavy paraffins. Current gasolines generally have aromatics contents of about 30% or higher, and may contain more than 40% aromatics.
Currently, refiners are faced with the prospect of supplying reformulated gasoline to meet tightened automotive emission standards. Reformulated gasoline would differ from the existing product in having a lower vapor pressure, lower final boiling point, increased content of oxygenates, and lower content of olefins, benzene and aromatics. The oxygen content of gasoline will be 2% or more in many areas. Gasoline aromatics content is likely to be lowered into the 20-25% range in major urban areas, and low-emission gasoline containing less than 15% aromatics is being advocated for some areas with severe pollution problems.
Since aromatics have been the principal source of increased gasoline octanes during the recent lead-reduction program, severe restriction of the aromatics content will present refiners with processing problems. Currently applicable technology includes such costly steps as recycle isomerization of light naphtha and generation of additional light olefins by FCC and isobutane by isomerization as feedstock to an alkylation unit. Increased blending of oxygenates such as methyl tertiary-butyl ether (MTBE) and ethanol will be an essential part of the reformulated-gasoline program, but feedstock supplies will become stretched. Novel processing technology is needed to support an effective program.
3. Related Art
Process combinations for the upgrading of naphtha to yield gasoline are known in the art. These combine known and novel processing steps primarily to increase gasoline octane, generally by producing and/or recovering aromatics needed to compensate for lead-antiknock removal from gasoline over a period of about 15 years.
U.S. Pat. No. 3,788,975 (Donaldson) teaches a combination process for the production of aromatics and isobutane using an "I-cracking" reaction zone followed by a combination of processes including catalytic reforming, aromatic separation, alkylation, isomerization, and dehydrogenation to yield alkylation feedstock. The paraffinic stream from aromatic extraction is returned to the cracking step. The gasoline pool is made up of isomerized product, aromatics and optionally alkylate. Donaldson does not disclose the present process combination, however. Even with the paraffinic alkylate in the gasoline pool, aromatics content is a high 38 volume % and the scheme of Donaldson would not achieve the present reduction in aromatics content at constant gasoline-product octane number.
A combination process including hydrocracking for gasoline production is disclosed in U.S. Pat. No. 3,933,619 (Kozlowski). High-octane, low-lead or unleaded gasoline is produced by hydrocracking a hydrocarbon feedstock to obtain butane, pentane-hexane, and C.sub.7 + hydrocarbons. Alternative embodiments are disclosed for upgrading pentanes and hexanes, and the C.sub.7 + fraction may be sent to a reformer along with cyclohexane from isomerization of hydrocracked C.sub.6 to yield an aromatics-rich product. The present process combination is not disclosed in Kozlowski, however, nor would it achieve the present reduction in aromatics content at constant octane number of the gasoline product.
U.S. Pat. No. 4,209,383 (Herout et al.) teaches a process combination for benzene reduction using catalytic reforming, catalytic cracking and alkylation of cracked light olefins with aromatics in the reformate. This scheme does not suggest the present combination nor does it result in an overall reduction in gasoline aromatics content.
U.S. Pat. No. 4,647,368 (McGuiness et al.) discloses a method for upgrading naphtha by hydrocracking over zeolite beta, recovering isobutane, C.sub.5 -C.sub.7 isoparaffins and a higher boiling stream, and reforming the latter stream. The reference neither teaches all the elements of nor suggests the present process combination, however.
The prior art, therefore, contains elements of the present invention. There is no suggestion to combine the elements, however, nor of the surprising benefits that accrue from the present process combination to produce a gasoline component for reformulated gasoline.